Coiled-coil unwinding at the smooth muscle myosin head-rod junction is required for optimal mechanical performance

Double-headed binding of myosin II to F-actin shows the effect of strain on head structure

Force production in muscle is achieved through the interaction of myosin and actin. Strong binding states in active muscle are associated with Mg·ADP bound to the active site; release of Mg·ADP allows rebinding of ATP and dissociation from actin. Thus, Mg·ADP binding is positioned for adaptation as a force sensor. Mechanical loads on the lever arm can affect the ability of myosin to release Mg·ADP but exactly how this is done is poorly defined. Here we use F-actin decorated with double-headed smooth muscle myosin fragments in the presence of Mg·ADP to visualize the effect of internally supplied tension on the paired lever arms using cryoEM. The interaction of the paired heads with two adjacent actin subunits is predicted to place one lever arm under positive and the other under negative strain. The converter domain is believed to be the most flexible domain within myosin head. Our results, instead, point to the segment of heavy chain between the essential and regulatory light chains as the location of the largest structural change. Moreover, our results suggest no large changes in the myosin coiled coil tail as the locus of strain relief when both heads bind F-actin. The method would be adaptable to double-headed members of the myosin family. We anticipate that the study of actin-myosin interaction using double-headed fragments enables visualization of domains that are typically noisy in decoration with single-headed fragments.

The myosin II coiled-coil domain atomic structure in its native environment

The atomic structure of the complete myosin tail within thick filaments isolated from Lethocerus indicus flight muscle is described and compared to crystal structures of recombinant, human cardiac myosin tail segments. Overall, the agreement is good with three exceptions: the proximal S2, in which the filament has heads attached but the crystal structure doesn't, and skip regions 2 and 4. At the head-tail junction, the tail α-helices are asymmetrically structured encompassing well-defined unfolding of 12 residues for one myosin tail, ∼4 residues of the other, and different degrees of α-helix unwinding for both tail α-helices, thereby providing an atomic resolution description of coiled-coil "uncoiling" at the head-tail junction. Asymmetry is observed in the nonhelical C termini; one C-terminal segment is intercalated between ribbons of myosin tails, the other apparently terminating at Skip 4 of another myosin tail. Between skip residues, crystal and filament structures agree well. Skips 1 and 3 also agree well and show the expected α-helix unwinding and coiled-coil untwisting in response to skip residue insertion. Skips 2 and 4 are different. Skip 2 is accommodated in an unusual manner through an increase in α-helix radius and corresponding reduction in rise/residue. Skip 4 remains helical in one chain, with the other chain unfolded, apparently influenced by the acidic myosin C terminus. The atomic model may shed some light on thick filament mechanosensing and is a step in understanding the complex roles that thick filaments of all species undergo during muscle contraction.

Post-ER Stress Biogenesis of Golgi Is Governed by Giantin

BACKGROUND

The Golgi apparatus undergoes disorganization in response to stress, but it is able to restore compact and perinuclear structure under recovery. This self-organization mechanism is significant for cellular homeostasis, but remains mostly elusive, as does the role of giantin, the largest Golgi matrix dimeric protein.

METHODS

In HeLa and different prostate cancer cells, we used the model of cellular stress induced by Brefeldin A (BFA). The conformational structure of giantin was assessed by proximity ligation assay and atomic force microscopy. The post-BFA distribution of Golgi resident enzymes was examined by 3D SIM high-resolution microscopy.

RESULTS

We detected that giantin is rather flexible than an extended coiled-coil dimer and BFA-induced Golgi disassembly was associated with giantin monomerization. A fusion of the nascent Golgi membranes after BFA washout is forced by giantin re-dimerization via disulfide bond in its luminal domain and assisted by Rab6a GTPase. GM130-GRASP65-dependent enzymes are able to reach the nascent Golgi membranes, while giantin-sensitive enzymes appeared at the Golgi after its complete recovery via direct interaction of their cytoplasmic tail with N-terminus of giantin.

CONCLUSION

Post-stress recovery of Golgi is conducted by giantin dimer and Golgi proteins refill membranes according to their docking affiliation rather than their intra-Golgi location.

Combined molecular/continuum modeling reveals the role of friction during fast unfolding of coiled-coil proteins

Coiled-coils are filamentous proteins that form the basic building block of important force-bearing cellular elements, such as intermediate filaments and myosin motors. In addition to their biological importance, coiled-coil proteins are increasingly used in new biomaterials including fibers, nanotubes, or hydrogels. Coiled-coils undergo a structural transition from an α-helical coil to an unfolded state upon extension, which allows them to sustain large strains and is critical for their biological function. By performing equilibrium and out-of-equilibrium all-atom molecular dynamics (MD) simulations of coiled-coils in explicit solvent, we show that two-state models based on Kramers' or Bell's theories fail to predict the rate of unfolding at high pulling rates. We further show that an atomistically informed continuum rod model accounting for phase transformations and for the hydrodynamic interactions with the solvent can reconcile two-state models with our MD results. Our results show that frictional forces, usually neglected in theories of fibrous protein unfolding, reduce the thermodynamic force acting on the interface, and thus control the dynamics of unfolding at different pulling rates. Our results may help interpret MD simulations at high pulling rates, and could be pertinent to cytoskeletal networks or protein-based artificial materials subjected to shocks or blasts.

A conformational transition in the myosin VI converter contributes to the variable step size

Myosin VI (MVI) is a dimeric molecular motor that translocates backwards on actin filaments with a surprisingly large and variable step size, given its short lever arm. A recent x-ray structure of MVI indicates that the large step size can be explained in part by a novel conformation of the converter subdomain in the prepowerstroke state, in which a 53-residue insert, unique to MVI, reorients the lever arm nearly parallel to the actin filament. To determine whether the existence of the novel converter conformation could contribute to the step-size variability, we used a path-based free-energy simulation tool, the string method, to show that there is a small free-energy difference between the novel converter conformation and the conventional conformation found in other myosins. This result suggests that MVI can bind to actin with the converter in either conformation. Models of MVI/MV chimeric dimers show that the variability in the tilting angle of the lever arm that results from the two converter conformations can lead to step-size variations of ∼12 nm. These variations, in combination with other proposed mechanisms, could explain the experimentally determined step-size variability of ∼25 nm for wild-type MVI. Mutations to test the findings by experiment are suggested.

Self-assembly of coiled coils in synthetic biology: inspiration and progress

Biological self-assembly is very complex and results in highly functional materials. In effect, it takes a bottom-up approach using biomolecular building blocks of precisely defined shape, size, hydrophobicity, and spatial distribution of functionality. Inspired by, and drawing lessons from self-assembly processes in nature, scientists are learning how to control the balance of many small forces to increase the complexity and functionality of self-assembled nanomaterials. The coiled-coil motif, a multipurpose building block commonly found in nature, has great potential in synthetic biology. In this review we examine the roles that the coiled-coil peptide motif plays in self-assembly in nature, and then summarize the advances that this has inspired in the creation of functional units, assemblies, and systems.

Common structural motifs for the regulation of divergent class II myosins

This minireview focuses on structural studies that have provided insights into our current understanding of thick filament regulation in muscle. We describe how different domains in the myosin molecule interact to produce an inactive "off" state; included are head-head and head-rod interactions, the role of the regulatory light chain, and the significance of the alpha-helical coiled-coil rod in regulation. Several of these interactions have now been visualized in a wide variety of native myosin filaments, testifying to the generality of these structural motifs across the phylogenetic tree.

Towards a unified theory of muscle contraction. I: foundations

Molecular models of contractility in striated muscle require an integrated description of the action of myosin motors, firstly in the filament lattice of the half-sarcomere. Existing models do not adequately reflect the biochemistry of the myosin motor and its sarcomeric environment. The biochemical actin-myosin-ATP cycle is reviewed, and we propose a model cycle with two 4- to 5-nm working strokes, where phosphate is released slowly after the first stroke. A smaller third stroke is associated with ATP-induced detachment from actin. A comprehensive model is defined by applying such a cycle to all myosin-S1 heads in the half-sarcomere, subject to generic constraints as follows: (a) all strain-dependent kinetics required for actin-myosin interactions are derived from reaction-energy landscapes and applied to dimeric myosin, (b) actin-myosin interactions in the half-sarcomere are controlled by matching rules derived from the structure of the filaments, so that each dimer may be associated with a target zone of three actin sites, and (c) the myosin and actin filaments are treated as elastically extensible. Numerical predictions for such a model are presented in the following paper.

The elastic properties of the structurally characterized myosin II S2 subdomain: a molecular dynamics and normal mode analysis

The elastic properties (stretching and bending moduli) of myosin are expected to play an important role in its function. Of particular interest is the extended alpha-helical coiled-coil portion of the molecule. Since there is no high resolution structure for the entire coiled-coil, a study is made of the scallop myosin II S2 subdomain for which an x-ray structure is available (Protein Data Bank 1nkn). We estimate the stretching and bending moduli of the S2 subdomain with an atomic level model by use of molecular simulations. Results were obtained from nonequilibrium molecular dynamics simulations in the presence of an external force, from the fluctuations in equilibrium molecular dynamics simulations and from normal modes. In addition, a poly-Ala (78 amino acid residues) alpha-helix model was examined to test the methodology and because of its interest as part of the lever arm. As expected, both the alpha-helix and coiled-coil S2 subdomain are very stiff for stretching along the main axis, with the stretching stiffness constant in the range 60-80 pN/nm (scaled to the 60 nm long S2). Both molecules are much more flexible for bending with a lateral stiffness of approximately 0.010 pN/nm for the S2 and 0.0055 pN/nm for the alpha-helix (scaled to 60 nm). These results are expected to be useful in estimating cross-bridge elasticity, which is required for understanding the strain-dependent transitions in the actomyosin cycle and for the development of three-dimensional models of muscle contraction.

An unstable head-rod junction may promote folding into the compact off-state conformation of regulated myosins

The N-terminal region of myosin's rod-like subfragment 2 (S2) joins the two heads of this dimeric molecule and is key to its function. Previously, a crystal structure of this predominantly coiled-coil region was determined for a short fragment (51 residues plus a leucine zipper) of the scallop striated muscle myosin isoform. In that study, the N-terminal 10-14 residues were found to be disordered. We have now determined the structure of the same scallop peptide in three additional crystal environments. In each of two of these structures, improved order has allowed visualization of the entire N-terminus in one chain of the dimeric peptide. We have also compared the melting temperatures of this scallop S2 peptide with those of analogous peptides from three other isoforms. Taken together, these experiments, along with examination of sequences, point to a diminished stability of the N-terminal region of S2 in regulated myosins, compared with those myosins whose regulation is thin filament linked. It seems plain that this isoform-specific instability promotes the off-state conformation of the heads in regulated myosins. We also discuss how myosin isoforms with varied thermal stabilities share the basic capacity to transmit force efficiently in order to produce contraction in their on states.

Structures of smooth muscle myosin and heavy meromyosin in the folded, shutdown state

Remodelling the contractile apparatus within smooth muscle cells allows effective contractile activity over a wide range of cell lengths. Thick filaments may be redistributed via depolymerisation into inactive myosin monomers that have been detected in vitro, in which the long tail has a folded conformation. Using negative stain electron microscopy of individual folded myosin molecules from turkey gizzard smooth muscle, we show that they are more compact than previously described, with heads and the three segments of the folded tail closely packed. Heavy meromyosin (HMM), which lacks two-thirds of the tail, closely resembles the equivalent parts of whole myosin. Image processing reveals a characteristic head region morphology for both HMM and myosin, with features identifiable by comparison with less compact molecules. The two heads associate asymmetrically: the tip of one motor domain touches the base of the other, resembling the blocked and free heads of this HMM when it forms 2D crystals on lipid monolayers. The tail of HMM lies between the heads, contacting the blocked motor domain, unlike in the 2D crystal. The tail of whole myosin is bent sharply and consistently close to residues 1175 and 1535. The first bend position correlates with a skip in the coiled coil sequence, the second does not. Tail segments 2 and 3 associate only with the blocked head, such that the second bend is near the C-lobe of the blocked head regulatory light chain. Quantitative analysis of tail flexibility shows that the single coiled coil of HMM has an apparent Young's modulus of about 0.5 GPa. The folded tail of the whole myosin is less flexible, indicating interactions between the segments. The folded tail does not modify the compact head arrangement but stabilises it, indicating a structural mechanism for the very low ATPase activity of the folded molecule.

Myosin II isoforms in smooth muscle: heterogeneity and function

Both smooth muscle (SM) and nonmuscle class II myosin molecules are expressed in SM tissues comprising hollow organ systems. Individual SM cells may express one or more of multiple myosin II isoforms that differ in myosin heavy chain (MHC) and myosin light chain (MLC) subunits. Although much has been learned, the expression profiles, organization within contractile filaments, localization within cells, and precise roles in various contractile functions of these different myosin molecules are still not well understood. However, data supporting unique physiological roles for certain isoforms continues to build. Isoform differences located in the S1 head region of the MHC can alter actin binding and rates of ATP hydrolysis. Differences located in the MHC tail can alter the formation, stability, and size of the myosin thick filament. In these distinct ways, both head and tail isoform differences can alter force generation and muscle shortening velocities. The MLCs that are associated with the lever arm of the S1 head can affect the flexibility and range of motion of this domain and possibly the motion of the S2 and motor domains. Phosphorylation of MLC(20) has been associated with conformational changes in the S1 and/or S2 fragments regulating enzymatic activity of the entire myosin molecule. A challenge for the future will be delineation of the physiological significance of the heterogeneous expression of these isoforms in developmental, tissue-specific, and species-specific patterns and or the intra- and intercellular heterogeneity of myosin isoform expression in SM cells of a given organ.

Crystal structures of human cardiac beta-myosin II S2-Delta provide insight into the functional role of the S2 subfragment

Myosin II is the major component of the muscle thick filament. It consists of two N-terminal S1 subfragments ("heads") connected to a long dimeric coiled-coil rod. The rod is in itself twofold symmetric, but in the filament, the two heads point away from the filament surface and are therefore not equivalent. This breaking of symmetry requires the initial section of the rod, subfragment 2 (S2), to be relatively flexible. S2 is an important functional element, involved in various mechanisms by which the activity of smooth and striated muscle is regulated. We have determined crystal structures of the 126 N-terminal residues of S2 from human cardiac beta-myosin II (S2-Delta), of both WT and the disease-associated E924K mutant. S2-Delta is a straight parallel dimeric coiled coil, but the N terminus of one chain is disordered in WT-S2-Delta due to crystal contacts, indicative of unstable local structure. Bulky noncanonical side chains pack into a/d positions of S2-Delta's N terminus, leading to defined local asymmetry and axial stagger, which could induce nonequivalence of the S1 subfragments. Additionally, S2 possesses a conserved charge distribution with three prominent rings of negative potential within S2-Delta, the first of which may provide a binding interface for the "blocked head" of smooth muscle myosin in the OFF state. The observation that many disease-associated mutations affect the second negatively charged ring further suggests that charge interactions play an important role in regulation of cardiac muscle activity through myosin-binding protein C.

Electron tomography of fast frozen, stretched rigor fibers reveals elastic distortions in the myosin crossbridges

As a first step toward freeze-trapping and 3-D modeling of the very rapid load-induced structural responses of active myosin heads, we explored the conformational range of longer lasting force-dependent changes in rigor crossbridges of insect flight muscle (IFM). Rigor IFM fibers were slam-frozen after ramp stretch (1000 ms) of 1-2% and freeze-substituted. Tomograms were calculated from tilt series of 30 nm longitudinal sections of Araldite-embedded fibers. Modified procedures of alignment and correspondence analysis grouped self-similar crossbridge forms into 16 class averages with 4.5 nm resolution, revealing actin protomers and myosin S2 segments of some crossbridges for the first time in muscle thin sections. Acto-S1 atomic models manually fitted to crossbridge density required a range of lever arm adjustments to match variably distorted rigor crossbridges. Some lever arms were unchanged compared with low tension rigor, while others were bent and displaced M-ward by up to 4.5 nm. The average displacement was 1.6 +/- 1.0 nm. "Map back" images that replaced each unaveraged 39 nm crossbridge motif by its class average showed an ordered mix of distorted and unaltered crossbridges distributed along the 116 nm repeat that reflects differences in rigor myosin head loading even before stretch.

Molecular and phenotypic effects of heterozygous, homozygous, and compound heterozygote myosin heavy-chain mutations

Autosomal dominant familial hypertrophic cardiomyopathy (FHC) has variable penetrance and phenotype. Heterozygous mutations in MYH7 encoding beta-myosin heavy chain are the most common causes of FHC, and we proposed that "enhanced" mutant actin-myosin function is the causative molecular abnormality. We have studied individuals from families in which members have two, one, or no mutant MYH7 alleles to examine for dose effects. In one family, a member homozygous for Lys207Gln had cardiomyopathy complicated by left ventricular dilatation, systolic impairment, atrial fibrillation, and defibrillator interventions. Only one of five heterozygous relatives had FHC. Leu908Val and Asp906Gly mutations were detected in a second family in which penetrance for Leu908Val heterozygotes was 46% (21/46) and 25% (3/12) for Asp906Gly. Despite the low penetrance, hypertrophy was severe in several heterozygotes. Two individuals with both mutations developed severe FHC. The velocities of actin translocation (V(actin)) by mutant and wild-type (WT) myosins were compared in the in vitro motility assay. Compared with WT/WT, V(actin) was 34% faster for WT/D906G and 21% for WT/L908V. Surprisingly V(actin) for Leu908Val/Asp906Gly and Lys207Gln/Lys207Gln mutants were similar to WT. The apparent enhancement of mechanical performance with mutant/WT myosin was not observed for mutant/mutant myosin. This suggests that V(actin) may be a poor predictor of disease penetrance or severity and that power production may be more appropriate, or that the limited availability of double mutant patients prohibits any definitive conclusions. Finally, severe FHC in heterozygous individuals can occur despite very low penetrance, suggesting these mutations alone are insufficient to cause FHC and that uncharacterized modifying mechanisms exert powerful influences.

Silanized surfaces for in vitro studies of actomyosin function and nanotechnology applications

We have previously shown that selective heavy meromyosin (HMM) adsorption to predefined regions of nanostructured polymer resist surfaces may be used to produce a nanostructured in vitro motility assay. However, actomyosin function was of lower quality than on conventional nitrocellulose films. We have therefore studied actomyosin function on differently derivatized glass surfaces with the aim to find a substitute for the polymer resists. We have found that surfaces derivatized with trimethylchlorosilane (TMCS) were superior to all other surfaces tested, including nitrocellulose. High-quality actin filament motility was observed up to 6 days after incubation with HMM and the fraction of motile actin filaments and the velocity of smooth sliding were generally higher on TMCS than on nitrocellulose. The actomyosin function on TMCS-derivatized glass and nitrocellulose is considered in relation to roughness and hydrophobicity of these surfaces. The results suggest that TMCS is an ideal substitute for polymer resists in the nanostructured in vitro motility assay. Furthermore, TMCS derivatized glass also seems to offer several advantages over nitrocellulose for HMM adsorption in the ordinary in vitro motility assay.

The kinetic properties of smooth muscle: how a little extra weight makes myosin faster

The contractile properties of smooth muscle (SM) are often described as fast and slow, but the molecular basis for the diversity in contractile properties has yet to be fully elucidated. Studies have shown that the differences in the contractile parameters are seen at the level of the contractile proteins. Experiments have implicated both the splicing of the SM myosin heavy chain (MHC) and the SM myosin essential myosin light chain as possible molecular determinants of the contractile properties of SM. This communication will focus on the role of the 7 aa insert in the smooth muscle MHC in determining the contractile properties of SM and the possible mechanism by which this insert could alter the kinetics of the SM actomyosin ATPase.

In vitro sliding of actin filaments labelled with single quantum dots

We recently refined the in vitro motility assay for studies of actomyosin function to achieve rectified myosin induced sliding of actin filaments. This paves the way, both for detailed functional studies of actomyosin and for nanotechnological applications. In the latter applications it would be desirable to use actin filaments for transportation of cargoes (e.g., enzymes) between different predetermined locations on a chip. We here describe how single quantum dot labelling of isolated actin filaments simultaneously provides handles for cargo attachment and bright and photostable fluorescence labels facilitating cargo detection and filament tracking. Labelling was achieved with preserved actomyosin function using streptavidin-coated CdSe quantum dots (Qdots). These nanocrystals have several unique physical properties and the present work describes their first use for functional studies of isolated proteins outside the cell. The results, in addition to the nanotechnology developments, open for new types of in vitro assays of isolated biomolecules.

Does the S2 rod of myosin II uncoil upon two-headed binding to actin? A leucine-zippered HMM study

Myosin II, like many molecular motors, is a two-headed dimer held together by a coiled-coil rod. The stability of the (S2) rod has implications for head-head interactions, force generation, and possibly regulation. Whether S2 uncoils has been controversial. To test the stability of S2, we constructed a series of "zippered" dimeric smooth muscle myosin II compounds, containing a high-melting temperature 32-amino acid GCN4 leucine zipper in the S2 rod beginning 0, 1, 2, or 15 heptads from the head-rod junction. We then assessed the ability of these and wild-type myosin to bind strongly via two heads to an actin filament by measuring the fluorescence quenching of pyrene-labeled actin induced by myosin binding. Such two-headed binding is expected to exert a large strain that tends to uncoil S2, and hence provide a robust test of S2 stability. We find that wild-type and zippered heavy meromyosin (HMM) are able to bind by both heads to actin under both nucleotide-free and saturating ADP conditions. In addition, we compared the actin affinity and rates for the 0- and 15-zippered HMMs in the phosphorylated "on" state and found them to be very similar. These results strongly suggest that S2 uncoiling is not necessary for two-headed binding of myosin to actin, presumably due to a compliant point in the myosin head(s). We conclude that S2 likely remains intact during the catalytic cycle.

A mutant heterodimeric myosin with one inactive head generates maximal displacement

Each of the heads of the motor protein myosin II is capable of supporting motion. A previous report showed that double-headed myosin generates twice the displacement of single-headed myosin (Tyska, M.J., D.E. Dupuis, W.H. Guilford, J.B. Patlak, G.S. Waller, K.M. Trybus, D.M. Warshaw, and S. Lowey. 1999. Proc. Natl. Acad. Sci. USA. 96:4402-4407). To determine the role of the second head, we expressed a smooth muscle heterodimeric heavy meromyosin (HMM) with one wild-type head, and the other locked in a weak actin-binding state by introducing a point mutation in switch II (E470A). Homodimeric E470A HMM did not support in vitro motility, and only slowly hydrolyzed MgATP. Optical trap measurements revealed that the heterodimer generated unitary displacements of 10.4 nm, strikingly similar to wild-type HMM (10.2 nm) and approximately twice that of single-headed subfragment-1 (4.4 nm). These data show that a double-headed molecule can achieve a working stroke of approximately 10 nm with only one active head and an inactive weak-binding partner. We propose that the second head optimizes the orientation and/or stabilizes the structure of the motion-generating head, thereby resulting in maximum displacement.

Neck length and processivity of myosin V

Myosin V is an unconventional myosin that transports cargo such as vesicles, melanosomes, or mRNA on actin filaments. It is a two-headed myosin with an unusually long neck that has six IQ motifs complexed with calmodulin. In vitro studies have shown that myosin V moves processively on actin, taking multiple 36-nm steps that coincide with the helical repeat of actin. This allows the molecule to "walk" across the top of an actin filament, a feature necessary for moving large vesicles along an actin filament bound to the cytoskeleton. The extended neck length of the two heads is thought to be critical for taking 36-nm steps for processive movements. To test this hypothesis we have expressed myosin V heavy meromyosin-like fragments containing 6IQ motifs, as well as ones that shorten (2IQ, 4IQ) or lengthen (8IQ) the neck region or alter the spacing between 3rd and 4th IQ motifs. The step size was proportional to neck length for the 2IQ, 4IQ, 6IQ, and 8IQ molecules, but the molecule with the altered spacing took shorter than expected steps. Total internal reflection fluorescence microscopy was used to determine whether the heavy meromyosin IQ molecules were capable of processive movements on actin. At saturating ATP concentrations, all molecules except for the 2IQ mutant moved processively on actin. When the ATP concentration was lowered to 10 microm or less, the 2IQ mutant demonstrated some processive movements but with reduced run lengths compared with the other mutants. Its weak processivity was also confirmed by actin landing assays.

Visualization of an unstable coiled coil from the scallop myosin rod

Alpha-helical coiled coils in muscle exemplify simplicity and economy of protein design: small variations in sequence lead to remarkable diversity in cellular functions. Myosin II is the key protein in muscle contraction, and the molecule's two-chain alpha-helical coiled-coil rod region--towards the carboxy terminus of the heavy chain--has unusual structural and dynamic features. The amino-terminal subfragment-2 (S2) domains of the rods can swing out from the thick filament backbone at a hinge in the coiled coil, allowing the two myosin 'heads' and their motor domains to interact with actin and generate tension. Most of the S2 rod appears to be a flexible coiled coil, but studies suggest that the structure at the N-terminal region is unstable, and unwinding or bending of the alpha-helices near the head-rod junction seems necessary for many of myosin's functional properties. Here we show the physical basis of a particularly weak coiled-coil segment by determining the 2.5-A-resolution crystal structure of a leucine-zipper-stabilized fragment of the scallop striated-muscle myosin rod adjacent to the head-rod junction. The N-terminal 14 residues are poorly ordered; the rest of the S2 segment forms a flexible coiled coil with poorly packed core residues. The unusual absence of interhelical salt bridges here exposes apolar core atoms to solvent.

Actomyosin motility on nanostructured surfaces

We have here, for the first time, used nanofabrication techniques to reproduce aspects of the ordered actomyosin arrangement in a muscle cell. The adsorption of functional heavy meromyosin (HMM) to five different resist polymers was first assessed. One group of resists (MRL-6000.1XP and ZEP-520) consistently exhibited high quality motility of actin filaments after incubation with HMM. A second group (PMMA-200, PMMA-950, and MRI-9030) generally gave low quality of motility with only few smoothly moving filaments. Based on these findings electron beam lithography was applied to a bi-layer resist system with PMMA-950 on top of MRL-6000.1XP. Grooves (100-200nm wide) in the PMMA layer were created to expose the MRL-6000.1XP surface for adsorption of HMM and guidance of actin filament motility. This guidance was quite efficient allowing no U-turns of the filaments and approximately 20 times higher density of moving filaments in the grooves than on the surrounding PMMA.

Structural model of the regulatory domain of smooth muscle heavy meromyosin

The goal of this study was to provide structural information about the regulatory domains of double-headed smooth muscle heavy meromyosin, including the N terminus of the regulatory light chain, in both the phosphorylated and unphosphorylated states. We extended our previous photo-cross-linking studies (Wu, X., Clack, B. A., Zhi, G., Stull, J. T., and Cremo, C. R. (1999) J. Biol. Chem. 274, 20328-20335) to determine regions of the regulatory light chain that are cross-linked by a cross-linker attached to Cys(108) on the partner regulatory light chain. For this purpose, we have synthesized two new biotinylated sulfhydryl reactive photo-cross-linking reagents, benzophenone, 4-(N-iodoacetamido)-4'-(N-biotinylamido) and benzophenone, 4-(N-maleimido)-4'-(N-biotinylamido). Cross-linked peptides were purified by avidin affinity chromatography and characterized by Edman sequencing and mass spectrometry. Labeled Cys(108) from one regulatory light chain cross-linked to (71)GMMSEAPGPIN(81), a loop in the N-terminal half of the regulatory light chain, and to (4)RAKAKTTKKRPQR(16), a region for which there is no atomic resolution data. Both cross-links were to the partner regulatory light chain and occurred in unphosphorylated but not phosphorylated heavy meromyosin. Using these data, data from our previous study, and atomic coordinates from various myosin isoforms, we have constructed a structural model of the regulatory domain in an unphosphorylated double-headed molecule that predicts the general location of the N terminus. The implications for the structural basis of the phosphorylation-mediated regulatory mechanism are discussed.

The myosin coiled-coil is a truly elastic protein structure

Coiled-coils occur in a variety of proteins involved in mechanical and structural tasks in the cell. Their mechanical properties are important in various contexts ranging from hair elasticity to synaptic fusion. Beyond their importance in biology, coiled-coils have also attracted interest as programmable protein sequences for the design of novel hydrogels and materials. We have studied the elastic properties of the myosin coiled-coil at the single molecule level. The coiled-coil undergoes a massive structural transition at forces between 20 and 25 pN where the coil extends to about 2.5 times its original length. Unlike all other proteins investigated mechanically so far, this transition is reversible on a timescale of less than a second, making the coiled-coil a truly elastic protein.

Molecular mechanics of mouse cardiac myosin isoforms

Two myosin isoforms are expressed in myocardium, alphaalpha-homodimers (V(1)) and betabeta-homodimers (V(3)). V(1) exhibits higher velocities and myofibrillar ATPase activities compared with V(3). We also observed this for cardiac myosin from normal (V(1)) and propylthiouracil-treated (V(3)) mice. Actin velocity in a motility assay (V(actin)) over V(1) myosin was twice that of V(3) as was the myofibrillar ATPase. Myosin's average force (F(avg)) was similar for V(1) and V(3). Comparing V(actin) and F(avg) across species for both V(1) and V(3), our laboratory showed previously (VanBuren P, Harris DE, Alpert NR, and Warshaw DM. Circ Res 77: 439-444, 1995) that mouse V(1) has greater V(actin) and F(avg) compared with rabbit V(1). Mouse V(3) V(actin) was twice that of rabbit V(actin). To understand myosin's molecular structure and function, we compared alpha- and beta-cardiac myosin sequences from rodents and rabbits. The rabbit alpha- and beta-cardiac myosin differed by eight and four amino acids, respectively, compared with rodents. These residues are localized to both the motor domain and the rod. These differences in sequence and mechanical performance may be an evolutionary attempt to match a myosin's mechanical behavior to the heart's power requirements.

Orientation changes of the myosin light chain domain during filament sliding in active and rigor muscle

Structural changes in myosin power many types of cell motility including muscle contraction. Tilting of the myosin light chain domain (LCD) seems to be the final step in transducing the energy of ATP hydrolysis, amplifying small structural changes near the ATP binding site into nanometer-scale motions of the filaments. Here we used polarized fluorescence measurements from bifunctional rhodamine probes attached at known orientations in the LCD to describe the distribution of orientations of the LCD in active contraction and rigor. We applied rapid length steps to perturb the orientations of the population of myosin heads that are attached to actin, and thereby characterized the motions of these force-bearing myosin heads. During active contraction, this population is a small fraction of the total. When the filaments slide in the shortening direction in active contraction, the long axis of LCD tilts towards its nucleotide-free orientation with no significant twisting around this axis. In contrast, filament sliding in rigor produces coordinated tilting and twisting motions.

Holding two heads together: stability of the myosin II rod measured by resonance energy transfer between the heads

Myosin, similar to many molecular motors, is a two-headed dimer held together by a coiled-coiled rod. The stability of the coiled coil has implications for head-head interactions, force generation, and possibly regulation. Here we used two different resonance energy transfer techniques to measure the distances between probes placed in the regulatory light chain of each head of a skeletal heavy meromyosin, near the head-rod junction (positions 2, 73, and 94). Our results indicate that the rod largely does not uncoil when myosin is free in solution, and at least beyond the first heptad, the subfragment 2 rod remains relatively intact even under the relatively large strain of two-headed myosin (rigor) binding to actin. We infer that uncoiling of the rod likely does not play a role in myosin II motility. To keep the head-rod junction intact, a distortion must occur within the myosin heads. This distortion may lead to different orientations of the light-chain domains within the myosin dimer when both heads are attached to actin, which would explain previously puzzling observations and require reinterpretation of others. In addition, by comparing resonance energy transfer techniques sensitive to different dynamical time scales, we find that the N terminus of the regulatory light chain is highly flexible, with possible implications for regulation. An intact rod may be a general property of molecular motors, because a similar conclusion has been reached recently for kinesin, although whether the rod remains intact will depend on the relative stiffness of the coiled coil and the head in different motors.

The myosin power stroke

Optical trapping technology now allows investigators in the motility field to measure the forces generated by single motor molecules. A handful of research groups have exploited this approach to further develop our understanding of the actin-based motor, myosin, an ATPase that is capable of converting chemical energy into mechanical work during a cyclical interaction with filamentous actin. In this regard, myosin-II from muscle is the most well-characterized myosin superfamily member. By combining the data obtained from optical trap assays with that from ensemble biochemical and mechanical assays, this review discusses the fundamental properties of the myosin-II power stroke and, perhaps more significantly, how these properties are governed by this molecule's atomic structure and the biochemical transitions that define its catalytic cycle.

alpha beta Spectrin coiled coil association at the tetramerization site

On the basis of sequence homology studies, it has been suggested that the association of human erythrocytes alpha and beta spectrin at the tetramerization site involves interactions between helices. However, no empirical details are available, presumably due to the experimental difficulties in studying spectrin molecules because of its size and/or its structural flexibility. It has been speculated that erythrocyte tetramerization involves helical bundling rather than coiled coil association. We have used recombinant spectrin peptides to model alpha and beta spectrin to study their association at the tetramerization site. Two alpha peptides, Sp alpha 1-156 and Sp alpha 1-368, and one beta peptide, Sp beta 1898-2083, were used as model peptides to demonstrate the formation of the alpha beta complex. We also found that the replacement of R28 in Sp alpha 1-368 to give Sp alpha 1-368R28C abolished complex formation with the beta peptide. Circular dichroism techniques were used to monitor the secondary structures of the individual peptides and of the complex, and the results showed that both Sp alpha 1-156 and Sp beta 1898-2083 peptides in solution, separately, included helices that were not paired with other helices in the absence of their binding partners. However, in a mixture of Sp alpha 1-156 and Sp beta 1898-2083 and formation of the alpha beta complex, the unpaired helices associated to form coiled coils. Since the sequences of these two peptides that are involved in the coiled coil association are derived from a native protein, the information obtained from this study also provides insight toward a better understanding of naturally occurring coiled coil subunit-subunit association.